The present invention relates generally to the field of medical imaging and more specifically to the field of imaging dynamic, internal tissue, such as cardiac tissue, by computed tomography. In particular, the present invention relates to the generation of a signal representative of internal motion using projection data.
Computed tomography (CT) imaging systems measure the attenuation of X-ray beams passed through a patient from numerous angles. Based upon these measurements, a computer is able to reconstruct images of the portions of a patient's body responsible for the radiation attenuation. As will be appreciated by those skilled in the art, these images are based upon separate examination of a series of angularly displaced projection images. A CT system processes X-ray projection data to generate 2D maps of the line integral of linear attenuation coefficients of the scanned object at multiple view angle positions. These data are then reconstructed to produce an image, which is typically displayed on a monitor, and may be printed or reproduced on film. A virtual 3-D image may also be produced by a CT examination.
CT scanners operate by projecting fan shaped or cone shaped X-ray beams from an X-ray source. The X-ray beams may be collimated to control the shape and spread of the beams. The X-ray beams are attenuated as they pass through the object to be imaged, such as a patient. The attenuated beams are detected by a set of detector elements. Each detector element produces a signal affected by the attenuation of the X-ray beams, and the data are processed to produce signals that represent the line integrals of the attenuation coefficients of the object along the X-ray paths. These signals are typically called “projection data” or just “projections”. By using reconstruction techniques, such as filtered backprojection, useful images may be formulated from the projections. The images may in turn be associated to form a volume rendering of a region of interest. The locations of regions of interest, such as pathologies, may then be identified either automatically, such as by a computer-assisted detection (CAD) algorithm or, more conventionally, such as by a trained radiologist. CT scanning provides certain advantages over other types of techniques in diagnosing disease, particularly because it illustrates the accurate anatomical information about the body. Further, CT scans may help physicians distinguish between types of abnormalities more accurately.
CT imaging techniques, however, may present certain challenges when imaging dynamic internal tissues, such as the heart. For example, in cardiac imaging, the motion of the heart causes inconsistencies in the projection data, which, after reconstruction, may result in various motion-related image artifacts such as blurring, streaking, or discontinuities. In particular, artifacts may occur during cardiac imaging when projections that are not acquired at the same point in the heart cycle, i.e., the same phase, are used to reconstruct the image or images that comprise the volume rendering.
To avoid the image artifacts associated with cardiac motion, therefore, it is desirable to reconstruct projection data acquired at the same phase into the desired images. This may be done by gating the acquisition of the projection data (prospective gating), or by methods that cull the projection data after acquisition (retrospective gating). For example, retrospective gating refers to selecting the projection data acquired at the same cardiac phase for image reconstruction. Typically, a simultaneously acquired electrocardiogram (ECG) signal is used to select projection data at a common phase of cardiac motion. Prospective gating refers to modulating data acquisition, such as the X-ray tube output and projection data acquisition, in response to real-time measurement and analysis of the ECG signal, i.e., acquiring only the projections of interest at a specified phase of cardiac motion for reconstruction.
However, an ECG signal is a measure of the depolarization and repolarization of the cardiac muscle tissue, and does not actually represent the motion of the tissue. While the electrical cardiac events measured by an ECG are generally indicative of cardiac muscle contraction and motion, the ECG is still only an indirect indicator of cardiac motion. Because of the indirect nature of this relationship, artifacts may still be present in the images reconstructed using gating techniques that rely upon ECG signals. It is desirable, therefore, to devise an indicator or signal that is directly related to cardiac motion for use in gating techniques or other techniques that would benefit from knowledge of the actual internal motion.